1. Field of the Invention
The invention is generally related to the installation of decks on offshore structures and more particularly to the assembly of floating offshore structures.
2. General Background
In the offshore drilling industry, unlike ships which can be fully assembled at an inshore facility, many types of oil drilling or production facilities require part of the assembly to take place either at the field location itself or afloat prior to towing to the installation site. For example, it is, by the nature of the design of jacket type production platforms, that the production deck (topsides) be installed after the jacket has been installed and piled to the sea floor. The topsides are typically installed in one or more pieces using heavy lift marine cranes. This can be a costly and weather sensitive operation. Also, additional cost is incurred due to the additional logistical support for the final hook-up of the topsides to the platform which must then take place offshore. The offshore hook-up problem is further aggravated if the topsides requires several lifts and/or the production platform is in a remote location.
Concrete GBS production platforms and floating production platforms such as Spar Platforms can provide the option, due to their buoyancy capacity, of avoiding the cost associated with offshore heavy lift operations by allowing a "float over" deck installation operation. Using this prior art method, a fully completed deck is loaded on barges in a catamaran configuration, the platform is ballasted down to a reduced freeboard, and the topsides floated over the platform. The platform is deballasted, thereby picking up the topsides and lifting it to the proper elevation above the water line. However, the transport of the topsides and the mating operation itself must take place in fairly benign conditions.
Due to the large draft of Spar type platforms, the traditional construction sequence, for steel hull Spars, involves the joining of structural sections in the horizontal position, followed by upending of the entire Spar hull to the vertical position. The structural sections may consist of either plated hull tank sections only, or a combination of plated tank and truss type sections. Such Spar type platforms are described in U.S. Pat. Nos. 4,702,321 and 5,558,467. As a consequence of a horizontal assembly and upending sequence, the topsides can only be installed after the upending operation and thus must take place in a location with substantial water depth. This can result, depending on geographical location, in either:
the topsides having to be installed offshore in a non-sheltered area, which means the deck transport and installation become weather sensitive operations; or possibly require a long tow of the fully assembled Spar to the production site, if the risk of an offshore deck installation is too high and the topsides must be installed in a sheltered location.
Pending U.S. patent application assigned Ser. No. 08/931,461 discloses a method for assembling a floating offshore structure wherein raising the hull to bring it into engagement with the deck structure is accomplished by winching or deballasting or a combination of both. The deballasting is accomplished through the use of control lines connected between the hull and a surface vessel. The control lines are used to inject air, from compressors on a support vessel, into ballast tanks to expel water ballast in the tanks.
U.S. Pat. No. 5,403,124 discloses a manner of installing a full sized deck upon a substructure wherein a semi-submersible vessel supporting a deck is ballasted down to lower the deck into engagement with the substructure or hull.
The problem with known deck installation systems, including the pending application referred to above, is that they do not provide a means for rapidly lifting the hull to a level where it will act as one body together with the deck. Unless this is done rapidly, the deck will bang repeatedly on the top of the hull and possibly cause damage until the hull develops sufficient buoyancy to raise it enough so that the two bodies (deck and hull) behave as one in the seaway. In order to achieve this change of displacement, water ballast pumps would need to be very large and these would have to be coupled with large diameter pipes to accommodate the flow. Alternatively, the use of air compressors alone would require very high capacity compressors.
It can be seen that the present state of the art in the installation of topsides on a floating offshore structure such as a Spar type hull includes shortcomings which have not been adequately addressed.